The primary focus of work in the Molecular Genetics Section continues to be on growth regulation in mammalian cells. The investigation of growth inhibitory activity detected in a DNA- mediated gene transfer system is emphasized within this framework. Molecular cloning of growth inhibitory sequences is being pursued by analysis of a cosmid library derived from WI38 human embryo fibroblast genomic DNA. From this library a single candidate plasmid, which contains a 7SL pseudogene, is being analyzed for both growth suppression activity and function as an origin of replication. Development of two new assays for transient growth regulation has been a major goal over the past year. The first assay employs fluorescence activated cell sorting (FACS) to identify the transfected cell subpopulation in conjunction with appropriate staining techniques to measure cell cycle characteristics of that subpopulation. The second assay involves the use of a new magnetic affinity cell sorting (MACS) method.